So-called opaquers, which are also referred to as opaque base masses are used in the covering of metallic tooth crowns or bridges. The opaquers serve to cover the metal structure of the dental restorations, in order that the dark metal can be prevented from showing through and consequently the colour of the natural teeth can be simulated by the relatively thin veneer which is subsequently applied. In addition to a high covering power, the opaquer must also have good adhesion both to the metal structure and to the veneer so that the final tooth restoration is stable in itself.
Dental opaquers are known from the state of the art.
Thus DE-C-33 32 179 describes photopolymerizable dental opaquers which contain a photopolymerization catalyst and a mixture of zirconium dioxide and titanium dioxide as pigment. The opaquer is applied to a metal structure as a thin layer and then cured with light. The metal structure covered with the opaquer is then veneered with plastic and cured for example to form a tooth crown. However, a satisfactory through-hardening of the opaquer is not achieved.
Known from DE-C-41 19 483 are pasty opaque base masses for ceramic and plastic veneers which are fired on at high temperature and which contain glass powder, plus zirconium dioxide as opacifier. Colour pigments are optionally added to the base masses in order to colour them.
It is also known from the state of the art to use bead polymerizates as a component of tooth filling compositions or of materials for producing prostheses.
Thus EP-B-11 186 describes bead polymerizates of viscous dimethacrylates which can be used as filler in pasty dental masses.
EP-B-11 735 and EP-B-11 734 describe pasty X-ray-opaque tooth filling masses which contain bead polymerizates provided with filler. Used as fillers are, for example, barium sulphate and silicon dioxide. However, coloured pigments are not present in the bead polymerizate.
Furthermore, EP-B-84 769 describes cross-linked bead polymerizates which in particular contain silicon dioxide as filler, but again contain no coloured pigments. The bead polymerizates are used as a component of tooth filling materials, being unsuitable for covering materials which are coloured in an undesired way, such as metals, since they are highly transparent.
Cross-linked bead polymerizates filled with fillers and their use as filling means in pasty dental masses are also known from EP-B-11 190. Coming into consideration as fillers are in particular magnesium hydroxide carbonate, titanium dioxide, barium sulphate, zirconium dioxide and silicon dioxide. However, neither the incorporation of coloured pigment into the bead polymerizates nor the use of the bead polymerizates as a component of dental opaquers are described.
Finally, U.S. Pat. No. 4,500,658 discloses X-ray-opaque acrylic bead polymerizates which can be used to prepare dental prostheses. The X-ray opacity of the polymerizates is effected by incorporated X-ray-opaque pigment. When the polymerizates are processed to form dental prostheses, curing takes place by heat and not by irradiation with light. Furthermore, it is also not disclosed to use the X-ray-opaque polymerizates as dental opaquers for covering metal structures in the preparation of crowns or bridges.
In the case of known opaquers which are cured by light, the particular problem occurs that, as a result of their high content of covering pigments, such as TiO.sub.2 and ZrO.sub.2, they only cure to an unsatisfactory extent when irradiated with light. This is a difficulty which naturally does not occur in the case of heat-curing materials. A reduction in the content of covering pigments is not possible since they are necessary for good covering power. Therefore two requirements, namely high covering power and complete curing by light, are to be met which are contrary to each other since the pigments prevent the penetration of light into deeper layers of the applied opaquer, thereby preventing a complete polymerization.
Thus it is known in the case of conventional light-curing paste opaquers that they often cure only on the top layer and are soft and unpolymerized underneath.
Another problem arising with conventional paste opaquers is that they tend to separate if stored for an extended period into a filler-rich and a filler-poor phase.
It is therefore the object of the invention to provide a dental light-curing opaquer which, despite a high covering power, can be cured by light even in deeper layers and which does not exhibit separation problems even if stored for an extended period.